As a vehicle driving system of the related art, various systems are known (e.g., see PTLs 1 to 3). Among them, a system in PTL 1 is configured such that two engines, a first engine and a second engine, are mounted as a driving source. When the necessary torque is small, only the first engine is operated, an output thereof is input into a transmission, and when the necessary torque is large, by additionally operating the second engine section, the outputs of both engines are synthesized and input into the transmission, whereby the necessary torque is produced under an optimal condition depending on the load situation to improve the fuel efficiency of a vehicle.
A system in PTL 2 is configured such that power of an engine (substantially, considered as two engines) having two pistons of different strokes is input into the transmission in parallel via a one-way clutch and is transmitted to an output shaft.